Strength members, reinforcing members, and structural members made of metal are used in automobiles and various machines. These members are required to have properties such as a high strength, a light weight, and a compact size. From in the past, these members have been manufactured by various working methods such as welding of press-worked parts, and punching or forging of thick plates. It is extremely difficult to further decrease the weight and size of members produced by these manufacturing methods. For example, in order to manufacture a welded part by partially overlapping press-worked panels and welding them, it is necessary to form portions of excess thickness referred to as flanges on the edges of the press-worked panels. The weight of welded parts unavoidably increases due to forming the portions of excess thickness.
The working method referred to as hydroforming (see Non-Patent Document 1, for example) forms a tube into a complicated shape by introducing a high pressure working fluid into the interior of a tube (which is a material being worked) disposed inside a mold, and the tube is deformed by expansion so that the outer surface of the tube conforms to the inner surface of the mold. Parts having a complicated shape are integrally formed by hydroforming without the need to form a flange. In recent years, hydroforming has been actively used for the manufacture of automotive parts with the objective of reducing the weight of automotive parts.
Hydroforming is a type of cold working. A material being worked having a high strength such as at least 780 MPa has insufficient ductility in cold working. Therefore, forming this material into an automotive part having a complicated shape by hydroforming is difficult. In addition, the manufacturing process for hydroforming typically requires the three steps of bending, preforming, and hydroforming, which makes the process relatively complicated. Furthermore, a hydroforming machine is large and relatively expensive.
The present applicant previously disclosed a bending apparatus in Patent Document 1. FIG. 8 is an explanatory view schematically showing this bending apparatus 0.
The bending apparatus 0 manufactures a bent member from a metal material 1 by the following steps.
(a) The metal material 1 is supported by a support unit 2 so that it can move in its axial direction.
(b) The metal member 1 which is supported by the support unit 2 is fed by a feed unit 3 from the upstream side to the downstream side so that it can undergo bending by the following step (c) downstream of the support unit 2 while being fed.
(c) A portion of the metal member 1 is rapidly heated to a temperature at which quench hardening is possible by an induction heating coil 5 disposed downstream of the support unit 2. The metal member 1 is rapidly cooled by a water cooling unit 6 disposed immediately downstream of the induction heating coil 5. A movable roller die 4 has at least one set of roll pairs 4a which can support the metal member 1 while feeding it. The movable roller die 4 is disposed downstream of the water cooling unit 6. By two-dimensionally or three-dimensionally varying the position of the roll pairs 4a, a bending moment is imparted to the heated portion of the metal member 1.
The bending apparatus 0 can form an automotive part as a one-piece member having a high strength of at least 780 MPa by simple steps using a relatively inexpensive forming machine.